Foot (unit)

==Definition==

===International foot===
The [[international yard and pound]] agreement of July 1959 defined the length of the international yard in the United States and countries of the [[Commonwealth of Nations]] as exactly 0.9144 [[meter]]s. Consequently, since a foot is one third of a yard, the international foot is defined to be equal to exactly 0.3048 meters. This was 2&nbsp;[[Parts per million|ppm]] shorter than the previous US definition and 1.7&nbsp;ppm longer than the previous British definition.<ref>{{cite web
 |url         = http://www.npl.co.uk/reference/faqs/on-what-basis-is-one-inch-exactly-equal-to-25.4-mm-has-the-imperial-inch-been-adjusted-to-give-this-exact-fit-and-if-so-when-(faq-length)
 |publisher   = [[National Physical Laboratory (United Kingdom)|National Physical Laboratory]]
 |title       = On what basis is one inch exactly equal to 25.4 mm? Has the imperial inch been adjusted to give this exact fit and if so when?
 |access-date  = July 24, 2012
|url-status  = dead
 |archive-url  = https://web.archive.org/web/20120807195334/http://www.npl.co.uk/reference/faqs/on-what-basis-is-one-inch-exactly-equal-to-25.4-mm-has-the-imperial-inch-been-adjusted-to-give-this-exact-fit-and-if-so-when-(faq-length)
 |archive-date = August 7, 2012
|df          = mdy-all
}}</ref>

The 1959 agreement concluded a series of step-by-step events, set off in particular by the [[British Standards Institution]]'s adoption of a scientific standard inch of 25.4 [[millimetre]]s in 1930.

====Symbol====
The IEEE standard symbol for a foot is "ft".<ref name=IEEE>{{cite web |title=Recommended Unit Symbols, SI Prefixes, and Abbreviations|url=https://www.ewh.ieee.org/soc/ias/pub-dept/abbreviation.pdf|access-date=April 7, 2021}}</ref> In some cases, the foot is denoted by a [[Prime (symbol)|prime]], often approximated by an [[apostrophe]], and the inch by a double prime; for example, 2{{nbsp}}feet 4&nbsp;inches is sometimes denoted as 2′{{nbsp}}4″.<ref name=CMOS>{{cite book| title = Chicago Manual of Style | edition = 17th | date = 2017 | publisher = University of Chicago Press | at = ¶ 10.66}}</ref>

=== Imperial units ===
In [[Imperial units]], the foot was defined as {{sfrac|1|3}}&nbsp;yard, with the yard being realized as a physical standard (separate from the standard meter). The yard standards of the different [[Commonwealth of Nations|Commonwealth]] countries were periodically compared with one another.<ref>See, for example, ''Report on the Comparisons of the Parliamentary Copies of the Imperial Standards with the Imperial Standard Yard and the Imperial Standard Pound and with each other during the Years 1947 to 1948'' (H.M.S.O., London, 1950). ''Report on the Comparisons of the Parliamentary Copies of the Imperial Standards with each other during the Year 1957'' (H.M.S.O., London, 1958).</ref> The value of the United Kingdom primary standard of the yard was determined in terms of the meter by the [[National Physical Laboratory (United Kingdom)|National Physical Laboratory]] in 1964 to be {{val|0.9143969|u=m}},<ref>{{cite journal |author1=Bigg, P. H. |author2=Anderton, Pamela |date=March 1964 |title=The United Kingdom standards of the yard in terms of the meter |url=http://www.iop.org/EJ/abstract/0508-3443/15/3/308/ |url-status=dead |journal=British Journal of Applied Physics |volume=15 |issue=3 |pages=291–300 |doi=10.1088/0508-3443/15/3/308 |bibcode=1964BJAP...15..291B |archive-url=https://archive.today/20120803014104/http://iopscience.iop.org/0508-3443/15/3/308 |archive-date=August 3, 2012 |access-date=May 16, 2009 }}</ref> implying a pre-1959 UK foot of {{val|0.3047990|u=m}}.

The UK adopted the international yard for all purposes through the [[Weights and Measures Act 1963]], effective January 1, 1964.<ref>[http://www.bailii.org/ew/cases/EWHC/Admin/2002/195.html Thoburn v Sunderland City Council &#91;2002&#93; EWHC 195 (Admin) (18 February 2002)]</ref>

===Survey foot===
When the international foot was defined in 1959, a great deal of survey data was already available based on the former definitions, especially in the United States and in [[India]]. The small difference between the survey foot and the international foot would not be detectable on a survey of a small parcel, but becomes significant for mapping, or when the [[State Plane Coordinate System|state plane coordinate system]] (SPCS) is used in the US, because the origin of the system may be hundreds of thousands of feet (hundreds of miles) from the point of interest. Hence the previous definitions continued to be used for surveying in the United States and India for many years, and are denoted '''survey feet''' to distinguish them from the international foot. The United Kingdom was unaffected by this problem, as the [[retriangulation of Great Britain]] (1936–62) had been done in meters.

====US survey foot{{anchor|United States survey foot}}====
In the United States, the foot was defined as 12&nbsp;inches, with the inch being defined by the [[Mendenhall Order]] of 1893 as 39.37&nbsp;inches&nbsp;= 1&nbsp;m (making a US foot exactly {{sfrac|1200|3937}}{{nbsp}}meters, approximately {{val|0.30480061|u=m}}).{{r|NYT 2020-08-18}}<ref>A. V. Astin & H. Arnold Karo (1959). [http://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdf "Refinement of values for the yard and the pound"]. {{webarchive|url=https://web.archive.org/web/20060821223520/http://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdf |date=August 21, 2006 }}. Washington DC: National Bureau of Standards. Republished on National Geodetic Survey web site and the Federal Register (Doc. 59-5442, filed June 30, 1959)</ref> Out of 50 states and six other jurisdictions, 40 have legislated that surveying measures should be based on the US survey foot, six have legislated that they be made on the basis of the international foot, and ten have not specified.<ref>[https://www.ngs.noaa.gov/SPCS/maps.shtml#spcs83 "State Plane Coordinate System"], National Geodetic Survey, May 4, 2019.</ref>

State legislation is also important for determining the conversion factor to be used for everyday land surveying and real estate transactions, although the difference (two [[parts per million]]) is of no practical significance given the precision of normal surveying measurements over short distances (usually much less than a mile).

The [[National Institute of Standards and Technology]], [[National Geodetic Survey]], and the [[United States Department of Commerce]] are phasing out the US survey foot beginning in 2023.<ref>{{Cite web| title = U.S. Survey Foot| work = National Institute of Standards and Technology| access-date = 2024-04-04| date = 2023-01-04| url = https://www.nist.gov/pml/us-surveyfoot}}</ref><ref>[https://www.ngs.noaa.gov/web/news/measure-unit-change-coming.shtml "Measuring Unit Change Coming in 2022"], National Geodetic Survey, June 14, 2019.</ref><ref name="NYT 2020-08-18">{{Cite news|last=Mitchell|first=Alanna|date=August 18, 2020|title=America Has Two Feet. It's About to Lose One of Them|language=en-US|work=The New York Times|url=https://www.nytimes.com/2020/08/18/science/foot-surveying-metrology-dennis.html|access-date=August 19, 2020|issn=0362-4331}}</ref> However, its relevance may persist, as the Federal Register Notice says:<ref>{{cite web | url=https://www.federalregister.gov/documents/2020/10/05/2020-21902/deprecation-of-the-united-states-us-survey-foot | website=Federal Register |title=Deprecation of the United States (U.S.) Survey Foot |date=October 5, 2020}}</ref>

{{quote|
The date of December 31, 2022, was selected to accompany the modernization of the National Spatial Reference System (NSRS) by [[National Oceanic and Atmospheric Administration|NOAA]]'s National Geodetic Survey (NGS). The reason for associating the deprecation of the U.S. survey foot with the modernization of the NSRS is that the biggest impact of the uniform adoption of the international foot will be for users of the NSRS, due to very large coordinate values currently given in U.S. survey feet in many areas of the U.S. Impacts related to the change to international feet will be minimized if a transition occurs concurrently with {{sic|others}} changes in the NSRS. ...

The difference in timelines will have no effect on users of the existing NSRS (National Spatial Reference System), because NGS ([[National Oceanic and Atmospheric Administration|NOAA]]'s National Geodetic Survey) ''will continue to support the U.S. survey foot for components of the NSRS where it is used now and in the past'' [emphasis added]. In other words, to minimize disruption in the use of U.S. survey foot for existing NSRS coordinate systems, the change will apply only to the modernized NSRS.}}

====Indian survey foot====
The Indian survey foot is defined as exactly {{val|0.3047996|u=m}},<ref>Schedule to the [https://web.archive.org/web/20091112174359/http://202.54.104.236/intranet/eip/legislation/uploads/THE%20STANDARDS%20OF%20WEIGHTS%20AND%20MEASURES%20ACT%201976.pdf Standards of Weights and Measures Act, 1976].</ref> presumably derived from a measurement of the previous Indian standard of the yard. The current National Topographic Database of the [[Survey of India]] is based on the metric [[WGS-84]] [[datum (geodesy)|datum]],<ref>[[Survey of India]], [http://www.surveyofindia.gov.in/tenders/nationalmappolicy/nationalmappolicy.pdf "National Map Policy – 2005"] {{webarchive|url=https://web.archive.org/web/20100331035549/http://www.surveyofindia.gov.in/tenders/nationalmappolicy/nationalmappolicy.pdf |date=March 31, 2010 }}.</ref> which is also used by the [[Global Positioning System]].